A new slimline high pressure, high-temperature tool (SlimXtreme HPHT) facilitated logging in a wellbore section with substantial mudcake buildup.
The Svane-1-1A exploratory well in the Danish Central Graben, off Denmark, drilled to 19,522 ft, set a new depth record, and produced gas and condensate from late Jurassic sandstones. Phillips Petroleum International Corp. Denmark (PPIC) and DONG Exploration & Production AS spudded the well in August 2001, evaluated it, and then plugged and abandoned it in August 2002.
Logging-while-drilling (LWD), wireline, and testing tools provided a wealth of information during the project. Because of the successful run of this tool to total depth, despite the difficult hole conditions and downhole environment encountered, Phillips is considering this technology for its next deep well in the Norwegian sector and is evaluating the use of these tools on future wells.
Drilling plan, results
Phillips received North Sea License 4/98 from the Danish Authorities in 1998 and immediately began formulating exploratory drilling plans (Fig. 1). Spudding date was set for Aug. 28, 2001.
Originally, the company planned to drill to 18,375 ft with 2.15 sp gr oil-based mud and a maximum 13° deviation. The geology includes Tertiary shales overlying a chalk interval, followed by lower Cretaceous and Jurassic shales. Phillips expected a reservoir section in Jurassic sandstones.
Project engineers designed a 121/4-in. hole to be drilled and cased just below the chalk before an expected pressure buildup. This section was to be logged with basic LWD logs and a single pass of porosity logs on wireline.
The 95/8 in. and 81/2-in. sections would follow to TD. Because of the substantial section of 81/2-in. hole, they designed a contingency plan that called for running a 71/2-in. casing liner and drilling to TD with 61/2-in. bits. The 81/2 in. and 61/2-in. sections, if eventually needed, were to be logged with wireline and LWD from TD to the 95/8-in. casing shoe.
At the wellsite, difficulties began in the 81/2-in. section. Substantial gas in the drilling mud slowed progress, so that a 71/2-in. contingency liner was run. Nonetheless, this part of the hole was subsequently lost and required redrilling.
Engineers initiated a sidetrack and set casing liner before drilling the final 61/2-in. hole to an extended TD of 19,522 ft. During the drilling process, the maximum temperature and pressure encountered were 179° C. and 18,000 psi, respectively.
Logging plan, results
The initial wireline logging plan required logging down at 6,000 fph to TD and back up at 1,800 fph, recording the repeat section after the main log. Regular-sized HPHT tools, however, were unable to log the last 328 ft of hole down to TD. Substantial mud cake buildup created havoc with the tools in the smaller hole sections.
Therefore, the Phillips team needed narrower HPHT tools to finish the job. The Svane-1 team, technically now the Svane-1A team, asked that a slim tool be made available in a short timeframe to complete the evaluation process.
Upon arrival at the wellsite in April 2002, the SlimXtreme tool was combined with a sonic sonde to create a "quad combo" toolstring that could acquire all of the final data needed in one run. This combined toolstring measured 102.9 ft in length by 3.0-in. OD. In this way, measurements taken in the deepest hole section would be comparable to those from the initial wireline runs.
Problems reaching TD on previous logging trips had raised concern about running standoffs on the slim toolstring, which are desirable to ensure centralization for the sonic and resistivity measurements. To maximize the chances of getting to bottom, however, the team only used the built-in 0.125-in. standoff. The run incorporated an in-line caliper to aid pad contact for the density and neutron porosity readings, while providing a measure of borehole diameter.
The slim tool successfully maneuvered the tough, narrow, HPHT conditions downhole and logged from TD up to the overlap with previous logging runs. The data from the two different toolstrings (regular and slim) repeated well at the overlap, validating the slim tool service run in combination with a sonic as a quad combo. The sonic and density sonde detection algorithms also performed well, despite lack of additional standoff.
All the wireline data from the deepest hole section were obtained in one run, as planned when the team employed the SlimXtreme service. Data quality was excellent and provided the final information needed for the PPIC subsurface team in Stavanger to fully evaluate the prospect.
As shown in Fig. 2, which includes the interval of interest, Track 1 reveals a slight washout on the caliper reading with a corresponding gamma ray that indicates sand. Also, an increase in resistivity in Track 2 indicates the possibility of hydrocarbons.
In Track 3, the crossover of the density and porosity curves gives a final indicator of hydrocarbons. Combined, these logging data focus in on a possible zone of interest and are the expected quality and reliability desired by ConocoPhillips. After logging to TD, two additional well tests were performed, prior to plugging and abandonment in August 2002.
Lessons learned
Despite the fact that Svane-1A was eventually plugged and abandoned, PPIC was able to thoroughly log and evaluate the hole. The operator's Stavanger personnel now view slimhole technology effective for deeper North Sea plays. The tool's ability to be combined with the sonic sonde to form a complete quad combo view of the wellbore is an added benefit.
The authors
Neil Anderson ([email protected]) has been an exploration petrophysicist for ConocoPhillips in Stavanger, Norway, for the past 7 years. He has also worked in Canada, California, Mexico, and the UK on various petrophysical projects. He was previously a field engineer with Schlumberger Ltd., a log analyst with Schlumberger and ResTech Houston Inc., and an independent consultant working in the UK and Norway. Most recently he has been involved with ConocoPhillips's HPHT wells drilled in the North Sea and Denmark. He graduated from the University of Saskatchewan (1975) in Mining Engineering.
David Cameron (dcameron2 @stavanger.oilfield.slb.com) is the international account manager for Schlumberger, Norway. He joined the company in 2000 after serving as a petroleum consultant with Independent Project Analysis Inc., an associate with ING Barings LLC, a field engineer with Western Atlas International Inc., and an offshore products design and development engineer with Avon Industrial Polymers Ltd. He holds an MBA from Erasmus University, Rotterdam, and a bachelor's of engineering from Brunel University, London.
